The shelf life of baked goods is mostly dependant on the rate of microbial spoilage. Both the characteristics of the food product itself, such as water activity (Aw), pH, redox potential, antimicrobial agents (preservatives), the chemical and biological composition of the baked good, as well as the environment in which it is stored (temperature, humidity and the gas composition of the package) affect the type and rate of microbial spoilage. Since baking of bread in an oven will kill all microorganisms with the exception of some spore forming bacteria, the contamination of bread by air-born mold spores after baking, during cooling and/or slicing of bread is the main source of microbial contamination. Furthermore, because most yeast raised baked goods such as bread, rolls, bagels, wheat flour tortillas, pizza crusts, donuts, croissants and pita breads are characterized by a relatively low water activity and a pH of around 5.0, mold is the main microorganism that causes spoilage of bread.
Traditionally, artificial additives have been added to baked goods to extend their shelf life. Currently, organic acids and their salts like calcium propionate are the main additives used for retarding molding of bread and bread-like products. Examples of the use of compounds such as propionates, sorbates and benzoates in baked goods to extend the shelf life of baked goods is well established in the art (See, e.g., U.S. Pat. Nos. 3,900,570, 4,416,904 and 6,123,973). Although propionates and related compounds are effective in retarding mold growth thereby extending the shelf life of the baked goods in which they are used, there are important limitations and disadvantages of using this chemical preservative.
First, even relatively low levels of these compounds can cause an undesirable off-taste, off-flavor, or off-smell. As one of skill in the art will appreciate, taste usually refers to the basic taste perceptions observed by the taste buds on the tongue, i.e., salt, sweet, bitter, acid and umami. Likewise, aroma refers to the perception from smelling a product with the many different receptors in our nose when the air containing volatile flavor compounds passes through our nose. When a food is eaten, a combination of taste and aroma is perceived. The combination of taste and aroma is perceived as flavor. This perception can be compared with that of a reference product. If the taste is different from expectations, e.g., too bitter, too salty, too acid, then the food can be described as having an off-taste. If the aroma is different from expectations, then the food can be described as having an off-smell. The combination of an off-taste or an off-smell can be described as off-flavor. If the food lacks sufficient taste or flavor, it can be described as bland.
Second, propionates not only inhibit the activity of yeast used for dough leavening, but also reduce the gas retaining capacity of dough resulting in lower loaf volume and an inferior crumb structure. Third, many countries limit the amount of propionate that can be used in baked goods. Finally, propionates are considered artificial chemicals and have been found less desirable by consumers as additives in bread with mainly natural label-friendly ingredients. While other chemical preservatives like sulfites, benzoates, sorbates, methyl and propyl parabens have also been used to some extent in baked goods, they are either less effective than propionates, interfere with the process of dough making, such as sorbates or sulfites that react during mixing with the disulfide bridges in the gluten protein structure, or their use is limited by regulation to baked goods other than bread.
Several attempts to overcome some of the limitations of using propionates and other chemical preservatives have been made by treating the surface of a baked good with a spray containing a preservative rather than incorporating it into the dough (see Pyler, Baking Science & Technology Vol I page 231, 1988 ISBN 0-929005-00-7). However, such methods have never found widespread usage because of the inherent difficulty in completely covering the surface of the baked good. Gaps in the coating provide areas where mold growth can still occur uninhibited. In addition to chemical preservatives, germicidal ultraviolet rays have been used for combating the molding problem in baked goods. However, this method is usually not sufficient to extend the mold-free shelf life of baked goods beyond the time for which such products are normally kept and requires considerable capital investments.
Modified atmospheric packaging (MAP) where the baked good is stored in special package with a low oxygen atmosphere has been the main alternative to extend mold-free shelf life without the use of chemical preservatives. However this method tends to be too expensive for most baked goods because it requires special packaging and special equipment for removing the oxygen from the baked good. The process works by (repeatedly) applying vacuum and flushing the packaged baked good with inert gasses like nitrogen and carbon dioxide gas to remove almost all of the oxygen remaining in the baked product. Among other disadvantages, this method cannot easily be used on high speed lines like bread lines without considerably reducing line speed and output.
Another known method for extending shelf life of food products including bread is the application of a glucose oxidase/catalase enzyme preparation on a food product stored in a sealed package in quantities high enough to reduce the level of oxygen in the package to under 1% within 5 days. (See, U.S. Pat. No. 4,996,062). A major disadvantage of this method is the high quantities of glucose oxidase required which makes the process cost-prohibitive from a commercial standpoint and can cause inhalation allergies when applied by spraying. The process also requires special packaging with low oxygen permeability to maintain a low level of residual oxygen in the package.
A further method and packaging for protecting food products from mold and oxidation is disclosed in U.S. Pat. No. 2,987,403. This method includes inoculating the food with an aerobic organism like yeast and sealing said food product in an oxygen impermeable enclosure like a can, tin jar or wrap. This reference does not disclose the application of the process to baked goods like bread and requires special packaging with low oxygen permeability unlike the conventional plastic polyethylene bags used to package bread and other baked goods. In general, low residual oxygen levels below 1% are required for stopping mold growth which can only be achieved by powerful oxygen absorbers in combination with special packaging to maintain a low level of residual oxygen in the packaged food product.
Another method for retarding mold growth is disclosed in U.S. Pat. No. 7,198,810. This reference discloses the use of a special yeast strain and/or process to increase the level of ethanol in dough (and the resulting bread) to a high level (0.8%-1.5%) as a means to retard mold growth. While high levels of alcohol in bread may help to retard mold growth, this method is difficult to implement since it requires major changes in process and recipe that will affect bread quality and will increase production costs substantially. Moreover, the gain in mold-free shelf life obtained by this method is relatively small.
The inoculation of microorganisms into baked goods is also known in the art. PCT Pub. No. WO 94/0019, for example, describes a method for adding live microorganisms with a potential health benefit to baked goods by injecting viable microorganisms, in particular probiotic lactic acid bacteria, in a protective matrix into the baked good. U.S. Pat. No. 6,835,397 discloses the protection and controlled release of fragile bioactive compounds including probiotic lactic acid bacteria and yeast for use in food and feed applications. Both references indicate the fragility of probiotic microorganisms in a food or feed product and the need to protect their viability not only during processing but also during subsequent storage of such a food or feed product.
Apart from molding there are other undesirable changes associated with bread staling occurring during storage of bread and other baked goods. During prolonged storage bread crumb becomes firmer while the freshly baked bread flavor gradually disappears. The introduction and increased usage of shelf life extending enzymes based on maltogenic amylase has greatly contributed to a solution for the crumb firming problem which has resulted in an increased need for more effective solutions to prevent molding and the loss of bread flavor. When bread is stored in conventional polyethylene plastic bags, much of the fermentation flavor will slowly dissipate through the plastic bag and disappear resulting in a bland flavor unless special packaging is used to prevent the loss of volatile compounds produced by yeast fermentation and by Maillard browning reactions during baking. While numerous patents disclose methods or compositions for improving and boosting the taste and flavor of freshly baked bread, these methods and compositions are normally not effective in preventing the loss of volatile flavor compounds during storage of bread. Consequently a great need exists for better and more cost-effective methods for preventing mold growth and counteracting the loss of freshly baked bread flavor during storage.
Many naturally occurring volatiles including acetaldehyde produced by yeast from ethanol in the presence of oxygen are known to have antifungal and antibacterial properties. The efficacy of plant volatiles including acetaldehyde for inhibiting the growth of decay microorganisms including molds has been reported by Made S.et al (J. Agric. Food Chem. 2002, 50, 6371-6377). However acetaldehyde is of limited use for this purpose because of its high volatility causing it to diffuse rapidly into the air. Almenar E. et al (J. Agric. Food Chem. 2007, 55(17), 7205-7212) describe an effective system to inhibit post harvest decay fungi based on a slow and controlled release of acetaldehyde from beta-cyclodextrins. An effective anti-molding system based on a slow release system to continously generate acetaldehyde is therefore be of considerable interest.